1. Field of the Invention
This invention relates to an improved process for the production of advanced NF.sub.4.sup.+ salts from NF.sub.4 SbF.sub.6. Advanced NF.sub.4.sup.+ salts are particularly useful for applications, such as solid propellant NF.sub.3 -F.sub.2 gas generators, high detonation pressure explosives, and fluorinating agents for aromatic compounds.
2. Description of the Prior Art
The NF.sub.4.sup.+ cation is unique since it combines high oxidizing power with unusual kinetic stability. Furthermore, its synthesis requires unconventional methods. Generally, complex fluoro cations are prepared by F.sup.- abstraction from a parent molecule. In the case of NF.sub.4.sup.+, however, the corresponding parent molecule, NF.sub.5, does not exist. The chemical synthesis of NF.sub.4.sup.+ from NF.sub.3 and F.sup.+ is precluded by the fact that fluorine is the most electronegative element known and, therefore, F.sup.+ cannot be generated by chemical means. This problem was overcome by the discovery that NF.sub.4.sup.+ salts can be prepared from NF.sub.3, F.sub.2 and a strong Lewis acid in the presence of an activation energy source. Although this principle has been demonstrated for several different Lewis acids and activation energy sources, such as heating, glow-discharge, bremsstrahlung and uv-photolysis, the only efficient and economically feasible method for preparing an NF.sub.4.sup.+ salt is based on the high temperature and high pressure reaction of NF.sub.3 and F.sub.2 with either SbF.sub.5 or SbF.sub.3. Consequently, the only readily accessible NF.sub.4.sup.+ salt is NF.sub.4.sup.+ SbF.sub.6.sup.-. Since the SbF.sub.6.sup.- anion is heavy and of low energy, it is undesirable for most practical applications and must be substituted by lighter or more energetic anions. Whereas the replacement of a weaker Lewis acid, such as BF.sub.3, by a stronger Lewis acid, such as PF.sub.5, AsF.sub.5 or SbF.sub.5, can be readily achieved by a simple displacement reaction, EQU NF.sub.4 BF.sub.4 +PF.sub.5 .fwdarw.NF.sub.4 PF.sub.6 +BF.sub.3
the reverse reaction, i.e. the displacement of SbF.sub.6.sup.- by BF.sub.3 is not possible. Based on the state of the art, substitution of SbF.sub.6.sup.- in NF.sub.4.sup.+ SbF.sub.6.sup.- is best achieved by metathetical processes in anhydrous HF, taking advantage of the relatively low solubility of alkali metal SbF.sub.6.sup.- salts. A typical example is the synthesis of NF.sub.4 BF.sub.4 from NF.sub.4 SbF.sub.6 according to: ##STR1## Among the major drawbacks of this process are: (i) the purity of the resulting NF.sub.4 BF.sub.4 product is only about 80-90%; (ii) the yield of NF.sub.4 BF.sub.4 is usually only 60 to 80% due to hang-up of mother liquor in the filter cake; (iii) cesium salts are expensive; (iv) the process must be carried out in a batch-type fashion; (v) the process requires low-temperature operations and cumbersome filtration steps.